There is known a spark plug in which a tip containing a noble metal is joined to a center electrode-facing surface of an electrode base of a ground electrode so as to improve the spark wear resistance of the ground electrode. For example, Japanese Laid-Open Patent Publication No. 2000-40577 discloses a technique of joining a tip to an electrode base by forming a weld zone from a surface of the electrode base opposite a facing surface thereof toward a bottom surface of the tip. In recent years, there has been a tendency to increase the size of tips in association with the progress of high boosting and high gas flow in combustion chambers for improvements in internal combustion engine efficiency and fuel efficiency.
In the technique of Japanese Laid-Open Patent Publication No. 2000-40577, however, the weld zone increases in size to ensure the joint strength of the tip as the size of the tip becomes increased. With such increase in weld zone size, there arises a possibility that the weld zone may be exposed at a surface of the electrode base other than the opposite surface. When a part of the weld zone is exposed at a surface of the electrode base other than the opposite surface, the exposed part of the weld zone can serve as a starting point of spark wear. This makes it likely that wear of the ground electrode will proceed.
The present invention addresses the above problem. An advantage of the present invention is a spark plug that has a ground electrode with a tip joined thereto and combines the joint strength of the tip with the spark wear resistance of the ground electrode.